Method and apparatus for blasting



Oct. 2, 1962 D. M. M FARLAND 3,056,349

METHOD AND APPARATUS FOR BLASTING Filed June 24, 1958 QUICK TRIP SWITCH ll l3 l5 INVENTOR.

David M. McFarland United dttes atent fire Patented Get. 2, 1962 3,656,349 METHGD AND AEPARATUS FQR BLASTING David M. McFarland, West (Disaster, Pa, assignor to Atlas Chemical industries, inc, Wilmingtcn, Del, carporstion of Delaware Filed June 24, 1958, er. 744,173 ill Claims. (Ql. 1ii2--22) This invention relates to a system of blasting and more specifically to a method and apparatus for initiating delay electric detonators, by the application of an electric current from a continuous source of high potential, such as a power line, in such a manner as to limit internal arcing and prevent consequent malfunctioning of the delay detonator.

The usual delay electric explosion initiator includes a metal shell having one end sealed with a plug of resilient material. Two leg Wires extend through the plug. The ends of the leg Wires within the shell portion are connected by a circuit that includes a short length of very fine Wire having a high electrical resistance, which is known as the bridge wire. Upon application of suflicient electric current, supplied through the leg wires, the bridge wire is heated. In some delay electric explosion initiators, the heated bridge Wire ignites a loose heat sensitive charge adjacent to the bridge wire, while in others the hot bridge wire ignites a solidified composition which had been applied to the bridge wire in a paste form. In either case the ignition action is intermediate and in turn ignites a delay element fabricated to burn a preset time interval and at the end of the interval to initiate an explosive charge located in the closed end of the metal shell.

As used herein the term initiate denotes the detonation or explosion phase in the operation of the detonator, and the term ignite denotes a slower burning phase such as the burning of the delay element within the detonator.

The use of delay electric detonators is well known in the blasting art. The most extensive use is to facilitate the detonation of explosive charges in rotation, or in so-called round firing.

It has been discovered that various malfunctions of delay electric detonators, which are seemingly unrelated, are caused by a large amount of heat, generated by an electrical are which occurs within the detonator when an excessive electrical current is supplied. The gases from the ignition composition within the detonator are heated rapidly and a high internal pressure is soon built up. The heat and high pressure condition within the detonator causes either an outward rupture of the detonator wall or a violent dislodging of the detonator seal, and the enclosed hot gases are rapidly released. The effect of this action on the blasting operation is unpredictable and varied. The sudden release of the gases may create conditions under which the burning delay element may be extinguished or the speed of burning of the delay element is retarded. The delay element will then fail to initiate the explosive charge of the detonator at the present time interval. The release of the hot gases and flame can ignite the surrounding explosive which may slowly burn with the possibility of detonation developing. If the delay element is extinguished and the surrounding explosive charge is ignited, several minutes may elapse before heat from the burning explosive either causes detonation or ignites the explosive charge of the detonator which initiates the remainder of the burning explosive. Heat generated by the burning explosive may decompose the explosive charge of the detonator, and, in such case, the surrounding explosive may, unpredictably, either burn completely or detonate at any point before it is completely consumed by burning. If the burning element is extinguished by the sudden release of the entrapped gases, and the explosive charge is not ignited by the hot gases, the shot may fail completely. Thus, the occurrence of a violent electrical are within the delay electric detonator may result in various malfunctions such as: a premature detonation, a misfire, or a delayed detonation commonly known as a hangfire.

The various malfunctions caused by arcing occur more frequently when any of the various parallel circuit arrangements such as normal parallel, reverse parallel, and closed loop parallel are used with a continuous source of electrical energy of high potential such as a power line. When electric detonators are connected in parallel, the voltage across the bridge wire terminals frequently is high enough to sustain an electrical are after the bridge wire melts and when a power line of high current capacity is used, the arc may generate enough heat within the detonator prior to initiation to cause malfunction. Parallel circuit arrangements of delay detonators are often used in the blasting art and the convenience and advantages of a power line as a source of electrical energy are Well known. Thus, the majority of arcing dilficulties encountered in blasting operations occur when a commonly used circuit arrangement is combined with a commonly used source of electrical energy.

As used herein, the term continuous as applied to a source of electrical energy suitable for carrying out the invention is meant to mean a source, such as a power line, which does not diminish significantly upon connection with a load. Such a source may supply either direct or alternating current.

According to the present invention, a succession of delay electric detonators may be fired in sequence with a high degree or" regularity at a predetermined time interval utilizing a continuous source of a high electrical potential such as a power line. A circuit is provided which consists of an arrangement of delay detonators and a quick trip switch connecting the detonator arrangement with a power line source of current. In the operaticn of the method of this invention the arrangement of delay detonators and the source of electrical current are connected for a single very short period of time.

A quantity of electrical energy is supplied through the quick trip switch to the delay electric detonator arrangement which is sufiicient to cause ignition of the detonators but insufficient to cause detrimental arcing therein.

The length of time taken to heat the bridge wire in a delay electric detonator, to a sufiiciently high temperature to cause ignition of the delay train, is a function of the current supplied. As compared to a series arrangement, the total current requirement of a parallel arrangement of detonators is relatively high as sufficient current must be supplied to fire each detonator independently. It is generally recommended by the manufacturers of a sence electrical detonators that when such detonators are used in parallel blasting arrangements not less than 0.75 ampere per detonator be supplied to the circuit, as below this amperage insufiicient electrical energy can be supplied to insure proper function of the detonators.

It has been discovered that the internal arcing of an electric detonator is also a function of the current and is directly related to the amount of electrical energy supplied per unit detonator. It can thus be stated that a minimum current is needed to properly ignite electric delay detonators arranged in parallel, and, further, to prevent misfires due to internal arcing, a maximum electrical energy value must not be exceeded.

In an electrical blasting circuit the electrical or heat energy per detonator may be expressed in watt-seconds. Mathematically watt-seconds (joules) per detonator may be expressed as:

where W=watt-seconds, I=the current applied in amperes per detonator, R=the bridge wire resistance per detonator in ohms and t=time the current is applied in seconds. The bridge wire resistance of commercial delay electric detonators marketed for succession or group firing is generally between 0.8 and 1.4 ohms, thus R in the above equation may usually be considered as ap proximately 1.

It has been found that the present invention is applicable when an electrical source having a potential in the range of 200 to 600 volts is employed. As previously explained a minimum current of about 0.75 ampere is generally needed to insure proper operation of electric detonators. Using calculated average currents of 0.6 to 50 amperes per detonator and potential values in the range of 200600 volts it has been discovered that there exists a very narrow range of time which the electrical source and a delay detonator arrangement may be connected to insure proper operation of the delay detonators. Expressed otherwise, it has been discovered that over relatively broad ranges of potential and current the amount of electrical energy that can be applied to each delay detonator in a blasting circuit to insure detonation without the danger of arcing misfires may be closely controlled by controlling the contact time of the electrical source and the detonator arrangement.

The ampere per detonator values referred to in this application are calculated average values obtained by dividing the total amperage of the blasting circuit by the total number of detonators in the circuit. For purposes of carrying out the invention these calculated average ampere per detonator values are adequate and may be readily obtained. The calculated average ampere per detonator value substituted in the Watt-second equation gives a calculated average watt-second value.

The calculated average energy values in watt-seconds per detonator which insure reliable detonations are usuall found to be between 0.008 and 30. Energy values below 0.008 watt-second have proved to be insufficient to reliably ignite a delay detonator, and energy values of over 30 have caused arcing and related misfires. The range of contact time desirable to be maintained between the electrical source and the detonator arrangement has been found to be at least 5 but not more than 200 milliseconds.

The invention will be further described in connection with the accompanying drawing which illustrates schematically and diagrammatically an embodiment of the invention, it being understood that the embod ment illustrated is susceptible of modification without departing from the spirit and scope of the appended claims.

Referring to the drawing: L and L represent the supply lines from a power line source of electrical energy. S and S represent lines which facilitate the connection of delay electric detonators 11, 13, 15, with power lines L and L A double pole quick trip switch 17 and a single throw double pole switch 19 are mounted on lines S and S between the delay detonator arrangement 11, 13, 15, and the point of contact between S and S with power lines L and L In operation the double pole quick trip switch 17 is set in a closed position and the single throw double pole switch 19 is used to complete the connection of the detonator arrangement 11, i3, 15 with the power line source L and L Upon the passage of electrical energy through quick trip switch 17, said switch operates to sever the electrical connection between the power line source L and L and the detonator arrangement 11, 13, 15. Thus, the interval of time during which electrical current is allowed to flow through the blasting circuit is very short and the electrical energy available to the detontor arrangement 11, l3, 15, is restricted to a single, short pulse.

The drawing demonstrates the most desirable blasting circuit coupling arrangement which includes a double pole switch 19 and a double pole quick trip switch 17. The two double pole switches represent a safety factor utilized in many blasting operations where both a safety and a firing switch are used. Although it is not a preferred form of the invention, double pole switch 19 may be eliminated from the blasting circuit and the quick trip switch 17 used to both connect and disconnect the detonator arrangement 11, 13, 15 with the electrical source L and L In order to lessen the hazard of stray ground current which may cause a premature detonation, switches used in blasting operations are generally of the two pole type. In blasting operations utilizing a power line source of current, the danger of stray ground current is greatly increased as the earth may act as a return or ground in place of the return side of the power line. Almost without exception, double pole switches are used in all blasting circuits utilizing a power line as a source of current. Mining regulations in many states make the use of double pole switches mandatory in this type of blasting operation. However, a single pole quick trip switch means would be suitable for carrying out the herein described nvention. Any switch means is suitable for use in this invention provided such means will disconnect Without again connecting an electrical circuit and a source of electrical current and will perform this function within a period of from 5 to 200 milliseconds after the application of electrical current.

Although a conventional circuit breaker may be used to perform the necessary make and break function, it is not usually a preferred means for the purpose. Generally a conventional circuit breaker is coupled between a supply circuit and a supplied circuit and operates to break the flow of current to the supplied circuit by monitoring the current flow through a series connected coil, and includes means to break the connection between the two circuits when the current level through the series coil rises above a preselected level; The performance of a series trip circuit breaker depends on the amperage drawn by the sup plied circuit. The amperage that will be drawn by a blasting circuit varies widely due to the length of each circuit, the size of Wire used, the number and type of detonators and the detonator arrangement. Thus, in order to function Within a specific and constant time interval such as from 5 to 200 milliseconds, the sensitivity of a conventional series trip circuit breaker would need to be manually set for each circuit to the amperage level in the supplied circuit. It Will be understood from the above discussion that a conventional circuit breaker may be used to perform the circuit make and break function utilized in this invention but such means is often far from satisfactory.

To produce a preferred means for performing the invention, a conventional circuit breaker was altered as described in detail in copending application 744,272 filed June 24, 1958, now US. Patent 3,019,315, by Albert A. Wolf for a Quick Trip Switch. The breaker device was adapted to break a circuit within a preset time interval, said circuit breaking action being substantially independent of the circuit load. The adaptation was made by mounting a solenoid in shunt across the load terminals of the circuit breaker and providing a means for opening the circuit contacts of the circuit breaker when the shunt solenoid is activated by an electrical potential. The sensitivity of the shunt coil to electrical potential may be adjusted so that the breaker means operates at a highly regular time interval over wide ranges of potential, eg 200 to 600 volts. The breaker thus functions when any potential of an ordinary use level is supplied to the circuit breaker and operates independently of the amperage drawn by the circuit.

In the preferred method of operation the altered circuit breaker is preset in a closed position. The power line source of electrical energy is connected to a double pole switch which is in turn connected through the circuit breaker to a parallel circuit arrangement of electric delay detonators. When the double pole switch is closed, the connection between the power line and the detonator circuit is completed. The potential impressed upon the millisecond delay detonator is initiated. Tests conducted using the millisecond type delay detonator to control the contact time are marked with an asterisk in the following table. In all cases where the detonator arrangement contains more than one detonator, the detonators were connected in parallel. In Test No. 1, for example, a single parallel arrangement consisting of ten commercial delay detonators were subjected to a calculated average current of 0.7 ampere per detonator for a time interval of 8 milliseconds. From the formula W=I Rt, using the value of 1 ohm as the electrical resistance of a single detonator, the electrical or heat energy in watt-seconds per detonator was calculated to be 0.004. The results of Test No. 1 show that because insufficient electrical energy was supplied to the detonator arrangement, none of the detonators in the arrangement functioned. In Test No. 8, three separate tests were conducted on parallel arrangements of ten detonators each. Each arrangement was subjected to a calculated average current of 1.0 ampere per detonator for a time interval of 8 milliseconds. The energy supplied per detonator was calculated to be 0.008 watt-second. The results in Test No. 8 show all of the detonators, comprising the three test groups, detonated properly. Tests 4, 5, 8, 9, 10, 11 and 13 are illustrative of the invention but should not be interpreted as limiting the invention thereto.

Example 1 Calculated Results Number Number Calculated Average of Detoof Average Contact Energy in Test Number meters in Arrange- Amperes Tin 1e m Watt- Failed Malfunc- Arrangements Per Mllll- Seconds Properly Due to tions ment Tested Detonator seconds Per Deto- Insufli- Due to Detonator nated eient Internal Energy Arcing 10 1 0.7 8 0. 004 0 10 0 10 1 0.8 8 0.005 0 10 0 1O 2 0.9 8 0. 006 0 20 0 10 2 1.0 8 0.008 20 0 0 5 2 21.0 8 3. 5 10 0 0 10 1 0.7 *7-10 0. 004 2 8 0 10 1 0.7 *7-10 0. 004 0 10 0 10 3 1.0 10 0.01 30 0 0 10 2 0. 7 *l832 0. 012 20 0 0 10 1 0. 6 *18-32 0. 009 10 0 0 1 5 10.0 *100 10 5 0 0 1 5 20. 0 *100 4 0 1 1 1 5 10.0 *200 20 5 0 0 1 3 20.0 *200 80 2 0 2 1 5 5 10 *300 30 20 0 2 5 1 5 9.0 *400 32 3 0 2 2 l 2 10.0 *400 40 1 0 z 1 2 1 10.0 *400 40 2 0 7t 2 1 2 20 *400 160 1 0 1 1 5 5 10 *400 40 17 0 2 8 5 5 5 *400 10 24 0 2 1 1 Failed to detonate.

2 Detonated but with shunt coil of the circuit breaker device will activate the opening mechanism and quickly break the connection. The time interval that the circuit remains closed may be regulated to a precise degree by adjusting the shunt coil or by increasing or decreasing a resistance which can be mounted in series with the shunt coil across the load terminals of the circuit breaker device.

Although it is not a preferred form of the invention, the altered circuit breaker may be used as a firing switch by either removing the aforementioned double pole switch from the circuit or by closing the double pole switch prior to closing the circuit contacts of the circuit breaker.

The following table is a compilation of various experiments conducted using commercial electric detonators. All tests shown in the table were conducted using a 200- 210 volt A.C. electrical source. The detonator arrangements were exposed to various conditions of current (amperes/detonator) and connected to the electrical source for various time intervals (contact time). Two methods of controlling the time interval were employed: (1) by means of a series coil circuit breaker device, and (2) by placing a millisecond type of delay electric detonator in the circuit so the circuit is ruptured when the slower than normal timing.

The following examples of tests conducted in the field are illustrative of the invention but should not be interpreted to limit the invention in any way.

Example 11 A surface test blast was conducted using 9 delay electric detonators and one instantaneous detonator connected in parallel using 250 total length of No. 14 copper leading wire. The detonator arrangement included one each of 1st through 7th and two 8th delay period detonators. Each detonator primed a half stick of 1%" X 8" gelatin explosive. The detonator arrangement was connected to a quick trip switch which in turn was connected through a double pole switch to a 440 volt A.C. source. At a point immediately in front of the switch the open-circuit voltage of the source measured 420 volts. The total blasting circuit resistance measured behind the switch was 1.23 ohms. Thus, there was a calculated current load of about 34 amperes per detonator. The quick trip switch was adjusted to operate and break the flow of electrical current approximately 25 milliseconds after electrical current was applied to the switch. When the circuit was completed through the quick trip switch, ten distinct rel ports were heard at nine regular intervals with both 8th period delay detonators firing almost simultaneously. An inspection of the blasting area after the blast revealed no unexploded detonators or powder.

Example Ill Comparative tests were conducted with and without a quick trip switch using a blasting circuit arranged as in Example II. In order to facilitate a careful study of the individual detonator action, no explosive charges were used in these comparative tests. Each detonator was packed in mud and covered to a depth of about 1".

In the test conducted without the quick trip switch, current was allowed to flow through the circuit for 10 seconds. Several reports were heard at irregular intervals. Later 3 unexploded detonators were recovered and an examination revealed that internal arcing had caused them to misfire.

In the test conducted with the quick trip switch the current was allowed to flow approximately 25 milliseconds. Ten reports were heard at nine regular intervals with both 8th period delay detonators firing almost together as in Example II. An inspection of the blast area and detonator remnants revealed that all detonations had been complete.

While a specific embodiment and examples of the invention have been described, it will be understood that the construction thereof and arrangement of the various parts may be altered without departing from the spirit and scope of the invention. Furthermore, the foregoing description should not be construed as limiting this invention to the specific embodiments illustrated, excepting as it may be limited in the appended claims.

What is claimed is:

1. A method of blasting which comprises applying a current from a continuous source of electrical potential between about 200 and about 600 volts to a parallel arrangement of delay electric detonators for a time interval sufficient to ignite said detonators and insuflicient to cause malfunction of the said detonators by internal arcing.

2. A method of firing an arrangement of delay electric detonators connected in parallel which comprises connecting said arrangement of delay electric detonators connected in parallel to a continuous source of electrical potential between about 200 and about 600 volts for an interval of between and 200 milliseconds and immediately thereafter disconnecting said arrangement.

3. A method of firing a parallel arrangement of delay electric detonators which comprises the application of at least 0.008 and not more than 30 watt-seconds per detonator of electrical energy from a continuous source of electrical potential between about 200 and about 600 volts to said arrangement of delay electric detonators connected in parallel, for a time interval suificient to ignite said detonators and insufiicient to cause malfunction of said detonators by internal arcing.

4. The method described in claim 3 where the electrical source is connected to the detonatorarrangement for a time interval of not longer than 200 milliseconds.

5. A blasting circuit comprising a parallel arrangement of delay electric detonators and means for connecting the detonators of said parallel arrangement of delay electric detonators to a continuous source of electrical potential between about 200 and about 600 volts for a time interval sufficient to ignite said detonators and insufficient to cause malfunction of the said detonators by internal arcing.

6. The circuit as described in claim 5 wherein the detonator arrangement and the electrical source are connected for a time interval of from 5 to 200 milliseconds.

7. The circuit as described in claim 5 wherein the connection between the parallel arrangement of delay electric detonators and the continuous source of electrical potential is by means of a quick trip switch.

8. A blasting circuit comprising a parallel arrangement of delay electric detonators and means for subjecting the detonators of said parallel arrangement of delay electric detonators to a current load from a continuous source of electrical potential between about 200 and about 600 volts for a time interval sufiicient to ignite said detonators and insufiicient to cause malfunction of the detonators by internal arcing, said detonators being subjected to electrical energy of between 0.008 and 30 watt-seconds per detonator.

9. The blasting circuit as described in claim 8 wherein the detonator arrangement and the electrical source are connected for a time interval not longer than 200 milliseconds.

10. The circuit described in claim 8 wherein the detonator arrangement and the continuous source of electrical potential are connected by means of a quick trip switch.

11. A blasting circuit which comprises a. parallel arrangement of delay electric detonators and means of subjecting the detonators of said parallel arrangement of delay electric detonators to a current load from a continuous source of electrical potential between about 200 and about 600 voltsfor a time interval of at least 5 but not longer than 200 milliseconds and at energy levels of between 0.008 and 30 watt-seconds per detonator.

References Cited in the file of this patent UNITED STATES PATENTS 2,546,686 Bickel Mar. 27, 1951 2,636,075 Miller Apr. 21, 1953 2,644,117 Schmitt et al June 30, 1953 2,703,053 Castel Mar. 1, 1955 2,725,822 Janelid D66. 6, 1955 2,736,260 Schlumberger Feb. 28, 1956 2,892,128 Wolf June 23, 1959 FOREIGN PATENTS 757,734 France Oct. 16, 1933 

